It depends what you're doing. If you're running a ruby web app server and talking to the database, all the io you're doing is most likely synchronous. In the one-server-per-cpu model, anyway.
It's similar with Ruby/RoR - yeah, they can do async. But not on their own. With unicorn server, you still get no threading and just a bunch of processes. With puma you can do threading (async cooperative really) - as long as you keep the configuration/code within the limits of what's allowed.
And due to the extra care needed whenever you do caching/storage things, I expect unicorn is still the king in RoR deployments. (GH uses that for example)